The invention provides bivalent binding agents of the general structure <；bd1>；-linker-<；bd2>； with affinity for a member of the TGF-β superfamily, where bd1 and bd2 are the same and are polypeptide binding domains having an affinity for the same member of the TGF-β superfamily； and, linker is an unstructured polypeptide sequence comprising SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39, SEQ ID NO: 40 or SEQ ID NO: 51 and having a number of amino acids greater than or equal to X/2.5, where X equals the shortest linear distance between: (a) the C-terminus of an isolated form of the binding domain that is located at the N-terminus of the linker and that is specifically bound to its ligand； and, (b) the N-terminus of an isolated form of the binding domain that is located at the C-terminus of the linker and that is specifically bound to its ligand.
